Significant variations in the twinning rate can be observed both in the long term and in the short term. These may be explained under the hypothesis that women who bear dizygotic twins tend to conceive more readily than other women, and the difference in waiting to conception has been estimated from national statistics to be about two months. If confirmed, the hypothesis would have implications for the physiology of reproduction. Data on about 50,000 pregnancies collected prospectively in the Collaborative Perinatal Project will permit a direct test of the hypothesis and more precise estimation of any difference in average waiting time. The mothers of dizygotic twins will be compared with matched controls with respect to intervals between births and, for those who reported that the current pregnancy was planned, the length of time required to conceive. The records are being prepared for analysis.